Method of gasifying carbonaceous material

ABSTRACT

A method and apparatus is disclosed for generating a clean, low Btu fuel gas by the reaction of a carbonaceous fuel limestone mixture with oxygen and steam in a downdraft fixed bed gasifier. The product gas and slag are discharged from the lower end of the gasifier after having passed through an incandescent char matrix in which contaminants such as sulfur are scrubbed from the gas. A portion of the hot product gas is withdrawn from the upper end of the gasifier to dry, preheat, and devolatilize freshly introduced fuel material. This gas which contains a heavy oily emulsion of tars and sulfur compounds is then reintroduced into the gasifier. The gasifier is capable of gasifying essentially any hydrocarbon material such as, refuse, lignite, anthracite, bituminous coal, coke, oil, liquid waste and many others, including manure and other farm waste.

FUEL

FINES OXIDIZER 5i Paine/owe; 3 ZONE 5 4 United States Patent 1 1 1 1 B3,920,417 Fernandes Nov. 18, 1975 [54] METHOD OF GASIFYING CARBONACEOUS2,657,124 10/1953 Gaucher 48/197 MATERIAL 2,662,006 12/1953 Eastwood...48/197 2,691,573 12/1950 Mayland 48/210 Inventor: J Henry Fernandes,Windsor, 2,830,883 4/1958 Eastman 48/206 Conn. 3,186,830 1/1965 Moore eta1. 75/43 [73] Assignee: Combustion Engineering, Inc.,

Windsor, Conn Primary ExamineF-S. Leon Bashore Assistant Examiner-PeterF. Kratz [22] Flled: June 1973 Attorney, Agent, or FirmFrederick A.Goettel, Jr. {211 App], No.: 375,220

[44] Published under the Trial Voluntary Protest [57] ABSTRACT Programon January 28, 1975 as document A method and apparatus is disclosed forgenerating a B 375,220- clean, 10w Btu fuel gas by the reaction of acarbonaceous fuel limestone mixture with oxygen and steam in ['52] US.Cl 48/197 R; 48/99; 48/71; a downdraft fixed bed gasifier The productgas and 8/ 8/ 48/211 slag are discharged from the lower end of thegasifier [5 Int. Cl.2 after having passed through an incandescent charma- 1 Field of Search trix in which contaminants such as sulfur arescrubbed 73, 95, from the gas. A portion of the hot product gas is with-1 202, 197 210, drawn from the upper end of the gasifier to dry, pre- I208, 209, 211 heat, and devolatilize freshly introduced fuel material.This gas which contains a heavy oily emulsion of tars 1 References Citedand sulfur compounds is then reintroduced into the UNITED STATES PATENTSgasifier. The gasifier is capable of gasifying essentially 2,042,9986/1936 Johnson 48/214 any hydrocarbon. material Such refuse 2,111,5793/1938 Winkler et a1. 48/203 thracite, bituminous coal, Coke, Oil,liquid Waste and 2,133,496 11/1933 Winkler et a1. 48/211 ny othincluding manure and other farm waste- 2,28l,562 5/1942 Ditto et al48/200 2,558,746 7/1951 Gaucher 48/206 8 Clams 4 Drawmg Flgul'esIGNITION STABILIZING GAS OFF-TAKE 38 9, g IGNITION LINE FROM 52 FILTER r2 g; mtmmmsz gRBON 55 54 'DUCING one 32 Z 5 3O 34 4! l H II-" II Q 35 2STEAM G 5 36 REDUCIN 2 8 ZONE m 37 5 IFLow/ E 46 42 g HOT Q PRODUCT 50g! GAS OUTLET US. Patent Ndv. 18, 1975 Sheet 1 of2 GAS OFF-TAKE 38 x-C-E\= I 3121; o 10 2o PREHEATING, zoNE I IGNITION 4 LINE 28 26 FINES 8FROM 52 2 I FILTER 2 2- Mu CARBON I. RED Cl I 56 54 g ZONLEI N6 '32 34 OI 2 STEAM REDUCING 8 ZONE STEAM \.37

iELow z 46 42 l p TTTTTTTTTT HOT f, PRODUCT I 4% 50 b GAS OUTLET FIG.

US. Patent Nov. 18, 1975 Sheet20f2 3,920,417

lFUEL FUEL PRETREATMENT FUEL STABILIZING Fe LIMESTONE GAS 55] I I 13,OFFTAKE 38 22 SIO CLEAN L HOT PRODUCT 1 OXIDIZER GAS 2O 52 62 FINES 2 3634 7 T L sTEAIvI a: 64

L. PRODUCT FILTER SLAG WATER OXlDlZER F l G. 2

68 66 SOLID LIQUID LIQUID FUEL FUEL FUEL 70 OXIDIZER OXIDIZER 1L STEAM 2STEAM 72 PRODUCT PRODUCT GAS GAS SLAG SLAG F l 3 FIG. 4

METHOD OF GASIFYING CARBONACEOUS MATERIAL BACKGROUND OF THE INVENTIONOne solution to the problem, particularly in the United States where thebasic proven energy reserves are predominantly coal, is the gasificationof coal to produce a low Btu fuel gas suitable for firing in a steamgenerator and similar devices. Generally speaking coal gasificationinvolves the reaction of coal, at high temperatures, with a gascontaining oxygen, and steam to produce a gas containing CO and H which,as indicated, is suitable for use as a fuel. Processes for achievingcoal gasification can be conveniently divided into three categories:

A. Fixed bed gasification where lump coal is supported on a grate or byother means and the flow of gas and coal may be countercurrent orconcurrent.

B. Fluidized bed gasification where crushed or fine coal is fluidized bythe gasifying medium, giving an expanded fuel bed that can be visualizedas boiling liquid.

C. Suspension or entrainment gasification where fine coal is suspendedin the gasifying medium. The particles moving with the gas eitherlinearly or in a vortex pattern.

Fixed bed gasifiers are felt by many to be the most attractive from thestandpoint of economics. Among the economically desirable features offixed bed gasifiers are the following:

1. Their inherent high carbon conversion ratio.

2. The fact that a low producer volume is required per unit of gasproduction.

3. The minimum of fuel preparation required.

Most common of the fixed bed type gasifiers is the counter-current orupdraft type where the coal moves downwardly within the gasifier and allof the gas produced passes upward and exits from the upper end. Anundesirable feature of such an arrangement is that the product gas, as aresult of passing over the freshly introduced coal, volatilizes thetars, sulfur compounds, and other contaminants within the coal and thuspasses from the gasifier containing a heavy oily emulsion of tars,sulfur compounds, etc. Such gas obviously must be subsequently processedto remove these contaminants before it can be successfully fired in apower generating unit.

The present invention makes use of technology which has been developedin connection with vertical shaft furnaces. Such furnaces were initiallydeveloped for use in continuously producing cast iron from iron ore orscrap metals. A typical shaft furnace is shown and described in US. Pat.No. 3,186,830, entitled, Melting Process of W. H. Moore and H. H.Kessler.

In prior art gasification systems the means for removal of contaminatingsulfur compounds from the product gas has primarily included processingthe gas after it has exited from the gasifier and been considerabiycooled. Typical of such desulfurization systems are 2 those utilizingsolvent extraction or dry extraction techniques. Regardless of thespecific process used such systems require expensive equipment which inturn increases the cost per unit Btu content of the product gas, andaccordingly makes the system competitvely less attractive.

Other prior art systems have suggested the introduction of limestoneinto the gasifier chamber along with the carbonaceous fuel, steam andoxidizer, in order to react the sulfur compounds found in the fuel withthe calcium in the limestone to form calcium sulfur compounds. Thecalcium sulfur compounds are then passed from the gasifier in a moltenstate along with the slag passing from the bottom of the gasifier. Theuse of limestone to accomplish this is suggested in the disclosure ofU.S. Pat. No. 2,830,883 to D. Eastman. Such an arrangement has not beenused in conjunction with counter-current fixed bed gasifiers because ofthe aforementioned disadvantage in that the product gas would pick upcontaminant just prior to exiting from the gasifier and thusinsufficient temperature or time would be available for the contaminantto react with the limestone.

SUMMARY OF THE INVENTION The present invention is directed to a methodof producing a clean low Btu fuel gas by the reaction of a carbonaceousfuel with free oxygen and steam in a downdraft fixed bed gasifier. Thematerial to be gasified is introduced at the upper end of the gasifierwhere it is preheated, dried and devolatilized by a stream of hot gasfrom the lower zones of the gasifier which has been drawn upward and iswithdrawn from the gasifier at the upper end thereof. This upward flowof gas also serves to maintain the ignition level of the carbonaceousfuel material at a predetermined level within the oxidation zone of thegasifier. The portion of gas withdrawn from the upper end of thegasifier, which contains the volatiles released by the carbonaceousmaterial, is re-introduced into the gasifier in the oxidation zone wherethe temperature is sufficiently high to effect thermal cracking of thevolatile to valuable hydrocarbon compounds.

As the carbonaceous fuel material moves downward through the gasifiershaft it passes successively through oxidation and reduction zones whereair or some other free oxygen containing gas, and steam, respectivelyare introduced into the gasifier. The resulting reactions convert thecarbonaceous fuel material and other reactants to an incandescent charmatrix extending to the bottom of the gasifier shaft, gaseous products(the makeup thereof depending upon the level of the gasifier) and amolten slag.

In a preferred embodiment limestone is introduced along with thecarbonaceous fuel material and is converted to a molten state along withthe slag. As the molten slag/limestone mixture and the product gas movedownward through the incandescent char matrix the calcium in thelimestone reacts with any sulfur contained in the gas to form sulfurcompounds which are subsequently removed along with the molten slag fromthe bottom of the gasifier. The product gas, which also passes from thelower end of the gasifier, is comprised principally of hydrogen andcarbon monoxide substantially free of any contaminating sulfur, NO, andparticulate matter, all of these contaminating substances having beenremoved either chemically or through scrubbing action as the product gasprogressed down through the incandescent char matrix in the lower end ofthe gasifier.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. 1 is a partially schematic,vertical cross sectional view of carbonaceous material gasifyingapparatus according to the invention.

FIG. 2 is a schematic flow diagram illustrating the principles ofoperation of the process of the invention.

FIG. 3 is a simplified, vertical cross sectional view of anotherembodiment of the gasifier.

FIG. 4 is a view similar to FIG. 3 of still a further embodiment of thegasifier.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 the gasifiercomprises a vertical shaft 2 having upper and lower end portions 4 and 6respectively and a water-cooled refractory lined inner wall 8.Communicating with the upper end 4 of the gasifier is a solid fuelintroduction unit 10. This unit is adapted to receive properly sizedsuitably prepared solid fuel from a fuel source 12 and to feed same at adesired rate to the interior of the gasifier shaft. As shownschematically in the drawing, the fuel feeding unit comprises a screwtype feeding system driven by a suitable power source such as electricmotor 14. In operation pressure in the upper end of the gasifier shaftis controlled to atmospheric pressure and thus a reasonably sealed fuelfeeding unit is sufficient to preclude gas leaks from the gasifierthrough the fuel feeding unit. Solid fuel is fed into the gasifierthrough fuel inlet opening at a rate sufficient to establish a bed offuel extending from the bottom of the gasifier to a level 18 which liesabove the fuel inlet opening 20.

Also communicating with the upper end 4 of the gasifier shaft 2 at apoint above the bed level 18 is an offtake duct 22 for withdrawing fromthe gasifier a portion of the gases produced therein. This off flow ofgases serves to preheat the newly introduced fuel and to stabilize theignition line within the gasifier. These features of the gasifier willbe more fully described below in connection with the operation of thegasifier.

A first series of tuyeres 24 for introducing into the gasifier a gascontaining free oxygen is located at a position underlying the fuelinlet opening 20. As shown there is provided a plurality of such tuyeres24 equally spaced about the outer periphery of the shaft 2. The tuyeres24 are supplied with the free oxygen containing gas by means of oxidizerwindbox or manifold 26 which extends circumferentialy around the shaftand is connected to a suitable supply 28 of such gas. This oxident gasis preferably preheated prior to introduction into the gasifier forreasons which, again, will be more fully set forth in the description ofthe operation of the gasifier.

Underlying the oxidizer windbox 26 is a second similar manifoldarrangment 30 for introducing superheated steam into the gasifierthrough a second series of tuyeres 32. As shown in the drawings theportion of gas withdrawn from the upper end of the gasifier shaftthrough duct 22 is reintroduced into the gasifier along with thesuperheated steam through this second series of tuyeres 32. Preferably,a steam aspirator 34 positioned in the steam supply line 36 will act todraw the desired quantity of fuel gas from the upper gas off take duct22. However, under certain operating conditions such as at startup itmay be necessary to supplement the action of the aspirator 34 by use ofa circulating fan 38 which is arranged in parallel with the gas off takeduct 22. The aspirator is of conventional design and includes a suctionport 35 to which the gas off take duct 22 is connected.

The lower end 6 of the gasifier shaft 2 extends downwardly into and issupported by a gas collector chamber 40 which includes a product gasoutlet 42 at the side thereof and a slag tap 44 in its lower end. Theattachment point 46 of the shaft 2 to the collector chamber 40 is suchas to provide structural support for the shaft and to provide a gastight seal therebetween. As is evident from the drawing the lower end ofthe shaft penetrates only partially into the gas collector 40; the spacefrom the end of the shaft to the walls of the collector is partiallyoccupied by a tier of refractory brick 50 or the like which are spacedso as to provide a plurality of openings 48 through which gas, generatedin the shaft, may pass to enter the outer region of the collectorchamber and from there exit from the gasifier. These bricks 50 alsoserve to provide lateral support to the extreme lower end of the bed ofcarbonaceous material which is maintained in the gasifier.

Referring still to FIG. 1, a fines injector 52 is located about midwaybetween the series of tuyeres for introducing gas 24 and the second setof tuyeres 32 for introducing steam. This injector comprises a screwfeed mechanism 54, a suitable drive unit 56, and is supplied with finesthrough conduit 58 which have been removed from the product gas by afilter 61. This arrangement is shown schematically in FIG. 2.

For the purpose of clarifying the description and thus the understandingof the operation of the gasifier of the present invention, the interiorof the gasifier shaft will be separated into four generally distinctregions: The preheating zone adjacent to the upper end of the shaft; theoxidation zone underlying the preheating zone and adjacent the first setof tuyeres; the carbon reducing zone which lies between the two sets oftuyeres; and the steam reducing zone which extends from the second setof tuyeres downward to the bottom of the gasifier.

In operation, material to be gasified is introduced into the gasifiershaft 2 by means of the fuel feeding unit 10 to establish the fuel bedat a level 18 within the gasifier. The bed of fuel rests directly on thebottom of the gas collector 40. Once the gasifier is in operation thelower end of the fuel bed becomes essentially an incandescent charmatrix through which the gas and liquid slag may pass. The material tobe gasified may be essentially a hydrocarbon fuel of any type, such as,for example, refuse, lignite, anthracite, bituminous coal, coke, oil,liquid waste and many others, including manure and other farm waste.Many of the aforementioned fuels would of course require preparationprior to introduction into the gasifier. Generally, the fuel materialshould not contain a great deal of material below /8 inch in size andalso it should not contain too many large lumps that would interferewith the speed of the gasification reactions. An upper size limit ofaround 6 inches with a preferred size of between /2 inch and 2 inches isconsidered to produce the best balance of conditions for speed ofreactions and for establishing the char matrix within the gasifiershaft. A further important requirement is that the fuel be of suchcompressive strength as to support the carbon matrix. Also, the moisturecontent of the material feed should not exceed 10 to 12%. Moisture inexcess of this range would interfere with temperature control within thegasifier. Such control being achieved by the introduction of steam aswill be subsequently further expanded upon. Accordingly, some shredding,briquetting and drying of the fuel material may be required prior tointroduction into the gasifier. In the case of liquid carbonceousmaterials it is necessary to simultaneously introduce sufficient solidmaterial to establish the required char matrix, or alternatively, anartificial matrix of refractory brick or the like may be establishedwithin the gasifier shaft. Such arrangements are shown schematically inFIGS. 3 and 4.

In the preferred embodiment a quantity of limestone (the term limestoneas used herein is meant to include any CaO yielding material) isintroduced along with the material to be gasified as shown at 31. Thelimestone acts to increase the fluidity of the slag formed in the lowerend of the reactor and also reacts with any sulfur contained in the fuelto form calcium-sulfur compounds which are then removed along with themolten slag.

Upon entry into the preheating zone of the gasifier thefuel andlimestone mixture is dried, preheated and devolatilized by passing aportion of the hot gas formed in the lower zones of the gasifierupwardly through the newly introduced material. This upward flow of gasis withdrawn from the gasifier through the gas offtake duct 22 andpreferably is recirculated back into the steam reducing zone of thegasifier in admixture with the superheated steam. The proportion of gaswithdrawn from the gasifier at this point may vary from around 5 percentto up to 50 percent. Fifty percent of the gas might be withdrawn understartup conditions when the char matrix is still being establishedwithin the gasifier. Under normal conditions the amount of gas withdrawnthrough the offtake is around to percent. As previously indicated, thepreferred arrangement for accomplishing this is to provide the suctionrequired to remove this gas by means of a steam aspirator 34. Thisarrangement is preferred for the reason that the gases withdrawn fromthe upper end of the gasifier contain tars and other contaminants suchas sulphur compounds which form a heavy oily emulsion which would tendto clog or impede the operation of a fan. However, under some operatingconditions such as at startup, the fan 38 is necessary to supplement thesteam aspirator and thus is provided in a parallel arrangement in thegas offtake duct 22.

The level of the fuel bed is maintained at a height 18 above the levelof the oxidizing tuyeres 24 so that the upflow of hot gases is able topreheat the fuel to its ignition temperature. As a result, as the fuelmoves downward and is exposed to the hot oxidizing gases which areinjected through the first set of tuyeres 24, ignition occursimmediately and the carbon and volatile matter' of the fuel is oxidizedto form water vapor and carbon dioxide. Also formed in this zone, byreaction of sulfur and nitrogen, with oxygen are sulfur oxide gases andminor amounts of nitrogen oxides, respectively.

The level at which ignition occurs, identified as the ignition line inFIG. 1, is preferably maintained at a level above the oxidizing tuyeres24. The ignition level is stabilized at the desired point by control ofthe degree of preheating and drying of the fuel which in turn isachieved by the quantity of offtake flow from the upper end of the shaftthrough the duct 22. Control of this flow is achieved by appropriateoperations of the aspirator 34 or the fan 38.

The oxidizer gas introduced through tuyeres 24 may be air, oxygenenriched air or pure oxygen. The use of oxygen in place of air or oxygenenriched air as the oxident will be predicated on the economics of theinstallation and the use being made of the product gas. Due to theabsence of other gases when pure oxygen is used as the oxident, a higherBtu product gas will be produced under these circumstances. However, ifoxygen or oxygen enriched air is used, it becomes necessary to add steamin the oxidation zone to dilute the reaction and prevent a violentreaction and local overheating of the system. Preferably the uppertemperature limit in this zone is controlled to around 3,200F. Such anarrangement is not difficult to accomplish if the economics of thesystem dictate the use of oxygen. Referring to FIG. 2 a quantity ofsteam from the steam supply 37 to the steam reducing zone may bediverted as needed through conduit 62 to the oxidation zone. Regulationof the quantity of the flow is dependent upon the temperature in theoxidation zone.

Passing from the oxidation zone the fuel, now converted to anincandescent char matrix, reacts with the gases (CO H O, SO and N0formed in the oxidation zone, reducing them to essentially carbonmonoxide, hydrogen and some hydrocarbon gases. Also passing from theoxidation zone is a slag formed by the molten limestone and ash of thefuel material. This slag drips over the incandescent char matrix in thereducing zone removing the nitrogen oxides which may have formed in theoxidation zone and the sulfur oxide compounds. As pointed out above, thesize of the fuel material used is important to ensure the formation of aporous char matrix bed in the gasifier shaft which will allow a freeflow of gases so that the various reactions described can completelytake place.

Passing downwardly from the carbon reducing zone the solid fuel andgases are reacted with steam introduced through the steam tuyeres 32. Inthis zone any carbon dioxide remaining is reduced to carbon monoxide andthe steam'reacts with carbon to form hydrogen, carbon monoxide and smallquantities of hydrocarbon gases. The temperature in this zone is reducedand controlled by control of the addition of steam to provide a gas exittemperature of around 2,100F. This temperature may be higher or lowerdepending upon what is required to ensure that the slag remains moltenin the lower part of the collector chamber 40 until it is removedthrough the slag tap 44. This temperature of and a few of theilluminants passes downwardlythrough the steam reducing zone, throughthe openings 48 in the brick tier 50 into the collector 40 and passes,from the gasifier through gas outlet 42. Referring now to FIG. 2, theproduct gas which as indicated is at a temperature of around 2,000F isfirst passed through a waste heat boiler 37 (steam supply of FIG. 1)where the process steam utilized in the gasifier is formed. Any excesssteam generated in this boiler 58 is passed via steam line 59 to a pointexternal to the gasifier system, where it may be used. From the wasteheat boiler 37 the gas passes to a second heat exchanger 60 where theoxidizer gas is preheated. From this heater and depending upon the useto which it is to be put, the gas may be cleaned by a gas filter as at61. The flow of gas in the circuit from the gas outlet 42 is maintainedby an induced draft fan 64 which supplies the product gas to thereceiving system. The fines removed from the product gas in the filter61 may be recirculated to the gasifier through recirculating line 58 tothe fines injector 52 where they are introduced into the gasifier in thecarbon reducing zone by means of the feeder mechanism 54.

With some fuels it may be desirable to add a quantity of iron orfeldspar to the fuel mixture in order to lower the slag fusiontemperature and keep the slag viscosity down. Iron also helps to absorbsulfur and acts as a catalyst in the gasification reactions. Thismaterial would be introduced into the fuel feed mechanism along with thefuel and limestone, as shown at 55.

Referring to FIGS. 3 and 4 two additional embodiments of the gasifierare shown. In FIG. 3 the gasifier is shown having both liquid 66 andsolid 68 fuel intr duction units. With such an arrangement the quantityof solid fuel introduced need only be sufficient to establish andmaintain the required char matrix, the balance of the fuel could be aliquid such as a high sulfur fuel oil.

The FIG. 4 arrangement is adapted to burn all liquid fuel introducedthrough the fuel introduction means 70. The porous char matrix iscreated upon an arrangement of refractory brick 72 within the gasifier.The liquid hydrocarbon carbonizes on the refractory brick to form thehot carbonaceous char. The operation and reactions which occur in theseadditional embodiments is substantially the same as with the solid fuelversion described in detail above.

It should be noted that while the vertical gasifier has been describedherein as being circular in cross section it should be understood thatit is not meant to limit it to such a configuration. As a matter of factit may be that for large scale gasification a shaft of rectangular crosssection is preferred. It has been shown that a circular gasifier havingan inside diameter of much more than 10 feet may experience problemswith gas penetration. However if the short dimension of a rectangularvessel is kept well under ten feet, so that there is no gas penetrationproblem, the rectangular unit can be made as long as desired.

While these preferred embodiments of the invention have been shown anddescribed, it will be understood that they are merely illustrative andthat changes may be made without departing from the scope of theinvention as claimed.

What is claimed is:

l. A method for producing a fuel gas from a carbonaceous material whichcomprises:

a. providing a substantially vertical shaft furnace having a preheatingzone adjacent the upper end of the furnace, an oxidation zone underlyingsaid preheating zone, and a reducing zone underlying said oxidation zoneand adjacent the lower end of the furnace;

b. introducing a solid carbonaceous material having sufficientcompressive strength to support a carbon matrix into said preheatingzone;

c. oxidizing said carbonaceous material in said oxidation zone byreacting said carbonaceous material with free oxygen to form anintermediate gaseous product, a char matrix containing carbon, andmolten slag;

d. reacting said intermediate gaseous products with steam and withcarbon from said char matrix in said reducing zone to form a product gascomprising principally hydrogen and carbon monoxide;

e. flowing said molten slag downward through said char matrix;

f. removing said slag through the bottom of said furnace shaft;

g. maintaining an upward flow of a portion of the gases formed in saidoxidation and reducing zones sufficient to maintainthe level of ignitionof said carbonaceous material in said oxidation zone at a predeterminedlevel, said upward flow of gas passing through said preheating zone andcontacting said carbonaceous material, whereby said material is dried,devolatilized and heated to a temperature just under its ignitiontemperature, the volatile matter driven from the carbonaceous materialbeing carried from said furnace along with said portion of said gasesand re-introducing said portion of the gases and volatile matter intosaid furnace in said reducing zone;

h. exhausting the balance of said product gas from the lower end of saidfurnace.

2. The method of claim 1 wherein said portion of the gases is introducedinto said furnace along with said steam.

3. The method of claim 1 including introducing limestone into saidpreheating zone along with said carbonaceous material, melting saidlimestone and flowing said limestone downward along with said slagthrough said char matrix in said oxidation and reducing zones, reactingany sulfur contained in said product gas with the limestone in said slagto form molten calcium-sulfur compounds, said calcium-sulfur compoundspassing from said furnace in admixture with said slag, the intimatecontact between the limestone/slag mixture and said downward flowinggases as they pass concurrently through said char matrix assuringremoval of substantially all sulfur contaminants from said product gasprior to exhausting the gas from the gasifier.

4. The method of claim 1 wherein the upward flow of the portion of thegases comprises between about 5 to 50 percent of the total gas producedin the gasifier.

5. The method of claim 4 wherein the upward flow of a portion of thegases comprises between about 15 to 25 percent of the total gas producedin the gasifier.

6. The method of claim 1 including introducing a liquid carbonaceousfuel into said preheating zone in addition to said solid carbonaceousmaterial.

7. The method of claim 1 including controlling the temperature withinsaid oxidation zone byadmixing steam with said oxygen.

8. The method of claim 3 including introducing small quantities of ironalong with said carbonaceous material and limestone.

1. A METHOD FOR PRODUCING A FUEL GAS FROM A CARBONACEOUS MATERIAL WHICHCOMPRISES: A. PROVIDING A SUBSTANTIALLY VERTICAL SHAFT FURNACE HAVING APREHEATING ZONE ADJACENT THE UPPER END OF THE FURNACE AN OXIDATION ZONEUNDERLYING SAID PREHEATING ZONE, AND A REDUCING ZONE UNDERLYING SAIDOXIDATION ZONE AND ADJACENT THE LOWER END OF THE FURNACE; B. INTRODUCINGA SOLID CARBONACEOUS MATERIAL HAVING SUFFICIENT COMPRESSIVE STRENGTH TOSUPPORT A CARBON MATRIX INTO SAID PREHEATING ZONE; C. OXIDIZING SAIDCARBONACEOUS MATERIAL IN SAID OXIDATION ZONE BY REACTING SAIDCARBONACEOUS MATERIAL WITH FREE OXYGEN TO FORM AN INTERMEDIATE GASEOUSPRODUCT, A CHAR MATRIX CONTAINING CARBON, AND MOLTEN SLAG; D. REACTINGSAID INTERMEDIATE GASEOUS PRODUCTS WITH STEAM AND WITH CARBON FROM SAIDCHAR MATRIX IN SAID REDUCING ZONE TO FORM A PRODUCT GAS COMPRISINGPRINCIPALLY HYDROGEN AND CARBON MONOXIDE; E. FLOWING SAID MOLTEN SLAGDOWNWARD THROUGH SAID CHAR MATRIX; F. REMOVING SAID SLAG THROUGH THEBOTTOM OF SAID FURNACE SHAFT,; G. MAINTAINING AN UPWARD FLOW OF APORTION OF THE GASES FORMED IN SAID OXIDATION AND REDUCING ZONESSUFFICIENT TO MAINTAIN THE LEVEL OF IGNITION OF SAID CARBONACEOUSMATERIAL IN SAID OXIDATION ZONE AT A PREDETERMINED LEVEL, SAID UPWARDFLOW OF GAS PASSING THROUGH SAID PREHEATING ZONE AND CONTACTING SAIDCARBONACEOUS MATERIAL, WHEREBY SAID MATERIAL IS DRIED, DEVOLATILIZED ANDHEATED TO A TEMPERATURE JUST UNDER ITS IGNITION TEMPERATURE, THEVOLATILE MATTER DRIVEN FROM THE CARBONACEOUS MATERIAL BEING CARRIED FROMSAID FURNACE ALONG WITH SAID PORTION OF SAID GASES AND RE-INTRODUCINGSAID PORTION OF THE GASES AND VOLATILE MATTER INTO SAID FURNACE IN SAIDREDUCING ZONE; H. EXHAUSTING THE BALANCE OF SAID PRODUCT GAS FROM THELOWER END OF SAID FURNACE.
 2. The method of claim 1 wherein said portionof the gases is introduced into said furnace along with said steam. 3.The method of claim 1 including introducing limestone into saidpreheating zone along with said carbonaceous material, melting saidlimestone and flowing said limestone downward along with said slagthrough said char matrix in said oxidation and reducing zones, reactingany sulfur contained in said product gas with the limestone in said slagto form molten calcium-sulfur compounds, said calcium-sulfur compoundspassing from said furnace in admixture with said slag, the intimatecontact between the limestone/slag mixture and said downward flowinggases as they pass concurrently through said char matrix assuringremoval of substantially all sulfur contaminants from said product gasprior to exhausting the gas from the gasifier.
 4. The method of claim 1wherein the upwaRd flow of the portion of the gases comprises betweenabout 5 to 50 percent of the total gas produced in the gasifier.
 5. Themethod of claim 4 wherein the upward flow of a portion of the gasescomprises between about 15 to 25 percent of the total gas produced inthe gasifier.
 6. The method of claim 1 including introducing a liquidcarbonaceous fuel into said preheating zone in addition to said solidcarbonaceous material.
 7. The method of claim 1 including controllingthe temperature within said oxidation zone by admixing steam with saidoxygen.
 8. The method of claim 3 including introducing small quantitiesof iron along with said carbonaceous material and limestone.